If you have been programming in a functional, declarative, or imperative style, shifting focus to object oriented programming (OOP) may feel a bit foreign.
An OOP approach asks the programmer to think about modeling a problem as one or more
objects that interact with one another, keep state, and act upon data.
Objects can represent real world entities (such as cars or cats) - or more abstract concepts (such as integers, vehicles, or mammals).
Each object becomes a unique instance in computer memory and represents some part of the overall model.
Classes are the definitions of new object types, and from which new
instances of objects are created.
They often bundle data with code or functions that operate on that data.
In this sense, classes are blueprints or sets of instructions from which many objects of a similar type can be built and used.
A complex program can have many classes, each building many different flavors of objects.
The process of building an object from a class is known as
instantiation (or creating an instance of the class).
A class definition in Python is straightforward:
class MyClass: # Class body goes here
Class fields (otherwise known as
data members, or
variables) can be added to the body of the class:
class MyClass: number = 5 string = "Hello!"
An instance (object) of
MyClass can be created and bound to a name by calling the class (in the same way a function is called):
>>> new_object = MyClass() # Class is instantiated and resulting object is bound to the "new_object" name. # Note: the object address 'at 0x15adc55b0' will vary by computer. >>> new_object <__main__.MyClass at 0x15adc55b0>
Class attributes are shared across all objects (or instances) created from a class, and can be accessed via
dot notation - a
. placed after the object name and before the attribute name:
>>> new_object = MyClass() # Accessing the class attribute "number" via dot-notation. >>> new_object.number 5 # Accessing the class attribute "string" via dot-notation. >>> new_object.string 'Hello!' # Instantiating an additional object and binding it to the "second_new_object" name. >>> second_new_object = MyClass() >>> second_new_object # Note: the object address "at 0x15ad99970" will vary by computer. <__main__.MyClass at 0x15ad99970> # Second_new_object shares the same class attributes as new_object. >>> new_object.number == second_new_object.number True
Class attributes are defined in the body of the class itself, before any other methods. They are owned by the class - allowing them to be shared across instances of the class. Because these attributes are shared, their value can be accessed and manipulated from the class directly. Altering the value of class attributes alters the value for all objects instantiated from the class:
>>> obj_one = MyClass() >>> obj_two = MyClass() # Accessing a class attribute from an object. >>> obj_two.number 5 # Accessing the class attribute from the class itself. >>> MyClass.number 5 # Modifying the value of the "number" class attribute. >>> MyClass.number = 27 # Modifying the "number" class attribute changes the "number" attribute for all objects. >>> obj_one.number 27 >>> obj_two.number 27
Having a bunch of objects with synchronized data at all times is not particularly useful.
Fortunately, objects created from a class can be customized with their own
instance attributes (or instance properties, variables, or fields).
As their name suggests, instance attributes are unique to each object, and can be modified independently.
The special "dunder method" (short for "double underscore method")
__init__() is used to customize class instances, and can be used to initialize instance attributes or properties for objects.
For its role in initializing instance attributes,
__init__() is also referred to as a
class constructor or
__init__() takes one required parameter called
self, which refers to the newly initialized or created object.
Data for instance attributes or properties can then be passed as arguments of
__init__(), following the
MyClass now has instance attributes called
As you can see, the two attributes have been assigned to the first and second indexes of the
location (a tuple) argument that has been passed to
location_y attributes for a class instance will now be initialized when you instantiate the class, and an object is created:
class MyClass: # These are class attributes, variables, or fields. number = 5 string = "Hello!" # This is the class "constructor", with a "location" parameter that is a tuple. def __init__(self, location): # This is an instance or object property, attribute, or variable. # Note that we are unpacking the tuple argument into two separate instance variables. self.location_x = location self.location_y = location # Create a new object "new_object_one", with object property (1, 2). >>> new_object_one = MyClass((1, 2)) # Create a second new object "new_object_two" with object property (-8, -9). >>> new_object_two = MyClass((-8, -9)) # Note that new_object_one.location_x and new_object_two.location_x two different values. >>> new_object_one.location_x 1 >>> new_object_two.location_x -8
Note that you only need to pass one argument when initializing
MyClass above -- Python takes care of passing
self when the class is called.
method is a
function that is bound to either the class itself (known as a class method, which will be discussed in a later exercise) or an instance of the class (object).
Methods that operate on an object (instance) need to be defined with
self as the first parameter.
You can then define the rest of the parameters as you would for a "normal" or non-bound function:
class MyClass: number = 5 string = "Hello!" # Class constructor. def __init__(self, location): # Instance properties self.location_x = location self.location_y = location # Instance method. Note "self" as first parameter. def change_location(self, amount): self.location_x += amount self.location_y += amount return self.location_x, self.location_y
Like attribute access, calling a method simply requires putting a
. after the object name, and before the method name.
The called method does not need a copy of the object as a first parameter -- Python fills in
class MyClass: number = 5 string = "Hello!" def __init__(self, location): self.location_x = location self.location_y = location def change_location(self, amount): self.location_x += amount self.location_y += amount return self.location_x, self.location_y # Make a new test_object with location (3,7) >>> test_object = MyClass((3,7)) >>> (test_object.location_x, test_object.location_y) (3,7) # Call change_location to increase location_x and location_y by 7. >>> test_object.change_location(7) (10, 14)
Class attributes can be accessed from within instance methods in the same way that they are accessed outside of the class:
class MyClass: number = 5 string = "Hello!" def __init__(self, location): self.location_x = location self.location_y = location # Alter instance variable location_x and location_y def change_location(self, amount): self.location_x += amount self.location_y += amount return self.location_x, self.location_y # Alter class variable number for all instances from within an instance. def increment_number(self): # Increment the 'number' class variable by 1. MyClass.number += 1 >>> test_object_one = MyClass((0,0)) >>> test_object_one.number 5 >>> test_object_two = MyClass((13, -3)) >>> test_object_two.increment_number() >>> test_object_one.number 6
In previous concept exercises and practice exercise stubs, you will have seen the
pass keyword used within the body of functions in place of actual code.
pass keyword is a syntactically valid placeholder - it prevents Python from throwing a syntax error for an empty function or class definition.
Essentially, it is a way to say to the Python interpreter, 'Don't worry! I will put code here eventually, I just haven't done it yet.'
class MyClass: number = 5 string = "Hello!" def __init__(self, location): self.location_x = location self.location_y = location # Alter instance variable location_x and location_y def change_location(self, amount): self.location_x += amount self.location_y += amount return self.location_x, self.location_y # Alter class variable number for all instances def increment_number(self): # Increment the 'number' class variable by 1. MyClass.number += 1 # This will compile and run without error, but has no current functionality. def pending_functionality(self): # Stubbing or placholding the body of this method. pass
Ellen is making a game where the player has to fight aliens.
She has just learned about Object Oriented Programming (OOP) and is eager to take advantage of what using
classes could offer her program.
To Ellen's delight, you have offered to help and she has given you the task of programming the aliens that the player has to fight.
Define the Alien class with a constructor that accepts two parameters
<y_coordinate>, putting them into
y_coordinate instance variables.
Every alien will also start off with a health level of 3, so the
health variable should be initialized as well.
>>> alien = Alien(2, 0) >>> alien.x_coordinate 2 >>> alien.y_coordinate 0 >>> alien.health 3
Now, each alien should be able to internally track its own position and health.
Ellen would like the Alien
class to have a
hit method that decrements the health of an alien object by 1 when called.
This way, she can simply call
<alien>.hit() instead of having to manually change an alien's health.
It is up to you if
hit() takes healths points to or below zero.
>>> alien = Alien(0, 0) >>> alien.health # Initialized health value. 3 # Decrements health by 1 point. >>> alien.hit() >>> alien.health 2
You realize that if the health keeps decreasing, at some point it will probably hit 0 (or even less!).
It would be a good idea to add an
is_alive method that Ellen can quickly call to check if the alien is... well... alive. 😉
<alien>.is_alive() should return a boolean.
>>> alien.health 1 >>> alien.is_alive() True >>> alien.hit() >>> alien.health 0 >>> alien.is_alive() False
In Ellen's game, the aliens have the ability to teleport!
You will need to write a
teleport method that takes new
y_coordinate values, and changes the alien's coordinates accordingly.
>>> alien.teleport(5, -4) >>> alien.x_coordinate 5 >>> alien.y_coordinate -4
Obviously, if the aliens can be hit by something, then they need to be able to detect when such a collision has occurred.
However, collision detection algorithms can be tricky, and you do not yet know how to implement one.
Ellen has said that she will do it later, but she would still like the
collision_detection method to appear in the class as a reminder to build out the functionality.
It will need to take a variable of some kind (probably another object), but that's really all you know.
You will need to make sure that putting the method definition into the class doesn't cause any errors when called:
>>> alien.collision_detection(other_object) >>>
Ellen has come back with a new request for you. She wants to keep track of how many aliens have been created over the game's lifetime. She says that it's got something to do with the scoring system.
>>> alien_one = Alien(5, 1) >>> alien_one.total_aliens_created 1 >>> alien_two = Alien(3, 0) >>> alien_two.total_aliens_created 2 >>> alien_one.total_aliens_created 2 >>> Alien.total_aliens_created # Accessing the variable from the class directly 2
Ellen loves what you've done so far, but she has one more favor to ask.
She would like a standalone (outside the
Alien() class) function that creates a
Alien() objects, given a list of positions (as
>>> alien_start_positions = [(4, 7), (-1, 0)] >>> aliens = new_aliens_collection(alien_start_positions) ... >>> for alien in aliens: print(alien.x_coordinate, alien.y_coordinate) (4, 7) (-1, 0)